The regulation of gene expression in eukaryotic cells has been studied in two different systems. Using nucleic acid hybridization techniques, earlier work provided a detailed analysis of the mRNA population in differentiating muscle cell cultures and demonstrated changes in coding capacity, complexity and abundance class distributions during myogenesis. Adenovirus type-2 infected HeLa cells have been used to study the regulation of viral gene expression. Methods to map the viral structural genes were developed and have been combined with new techniques for selecting and sizing viral specific mRNA transcripts. This has given an overview of the structural gene arrangement and transcription pattern within the viral genome. The structure of mRNA has been studied with regard to those parameters which modulate translational efficiency, utilizing defined prokaryotic mRNA transcripts, modified structurally in various ways, in eukaryotic cell-free translational systems. Eukaryotic ribosome binding sites on these prokaryotic mRNAs have been sequenced and compared to prokaryotic ribosome binding sites on the same mRNA, characterizing the common mRNA structure seen by both types of ribosomes.